The invention relates generally to fiber optic chemical sensors (FOCS) and more particularly to reservoir FOCS (e.g., U.S. Pat. No. 4,892,383 to Klainer, et al.), specifically measurement techniques, cell design, construction materials, membrane selection, and applications.
The basic FOCS design is a tip coated FOCS in which a reagent that specifically and sensitively interacts with the analyte of interest is placed or attached at the tip of an optical fiber. Fibers in the 100 to 1,000 micron diameter range are most often used. Unfortunately, because of the very small surface areas at their tips (8.times.10.sup.-5 to 8.times.10.sup.-3 cm.sup.2) it is difficult to get enough chemistry at the end of the fiber for reliable measurements, i.e. good signal to noise.
Several approaches have been used to get enough chemistry onto the tip of the fiber and thus overcome the surface area limitations, including: (i) surface amplification techniques, a unique type of covalent immobilization in which the sensing material is attached to the tip by a surface amplification polymer; (ii) imbedding the chemistry into a membrane placed at the tip of the fiber; (iii) large surface area porous glass fibers attached to the tip; and (iv) use of very sensitive chemistry, i.e. fluorescence reactions. In special situations, where the species to be measured is volatile, reservoir FOCS (liquid reagents) with gas permeable membranes have been used. Each of these has shown severe drawbacks. The surface amplification approach works but it is impossible to make several FOCS, either individually or by a batch process, which are similar. Imbedding the chemistry in the membrane results in such disadvantages as slower response times, "leaking" of the reagents and difficulty in making uniform systems. Porous glass fibers, especially the newer ones with the larger pores (&gt;300 .mu.m), appeared to be a big breakthrough, but the large buffering capacity of the glass is a major problem as is its ability to trap unwanted molecules. The use of fluorophores, in the past, has been considered a drawback because of bleaching problems. Finally reservoir cells have been very difficult to use and to make uniformly, and it is also burdensome to obtain reliable data.
Evanescent wave FOCS and side coated FOCS, particularly the multilayered FOCS where the reacting chemistry is sandwiched between the core and clad as described in U.S. Pat. No. 4,846,548 to Klainer are alternative FOCS designs where the sensing material can be placed on the sides of the fiber.
The need for high resolution, long active lifetimes and good reproducibility between sensors for a particular species indicates the desirability of an improved reservoir sensor. The sensing reagents are in liquid form and kept in the sensor by a species permeable membrane. All of the key elements of the sensor should be very accurately controlled. Ideally, reagent solution can be made with a very high degree of repeatability, the active volume can be precisely controlled, the field-of-view of the fiber is accurately known, and the size and permeability of the membrane can be held to good tolerances. Thus designs are needed which can incorporate all these features and advantages. The big drawback of the reservoir FOCS is size. It is about 1 cm in diameter compared to other FOCS which are 400 to 1,000 .mu.m.
Thus while reservoir FOCS would be particularly desirable for many applications, including sampling in an aqueous environment, present sensors are inadequate. U.S. Pat. Nos. 4,757,343 to Hirschfeld and 4,666,674 to Miller et al. show typical reservoir FOCS formed by attaching a capillary tube coaxially to the end of an optical fiber using a gas bubble or membrane to close the tube. This structure is difficult to assemble accurately, difficult to control and use and impossible to reproduce uniformly. Another type of FOCS, while not actually a reservoir type, is shown by U.S. Pat. No. 4,478,872 to Peterson et al. wherein a porous polymer jacket or envelope is placed at the end of a pair of fibers and encloses a fluorescent dye on a solid (particulate) support.
U.S. Pat. No. 4,892,383 to Klainer, et al provides an improved reservoir FOCS having a modular design. A modular reservoir cell body has an optical fiber precisely positioned in one end by means of a quick connect fiber optic connector and a semi-permeable membrane attached across the opposite end by a quick connect membrane retainer. While this design overcomes many of the prior problems, the design is not optimally suitable for all applications. The width of the cell body limits the size of the membrane which is the surface through which the species of interest passes into the cell. The material that flows into the cell leaves the cell by a reverse flow through the membrane. The fiber may only be positioned at one end of the cell. A more versatile configuration with larger membrane area, better flow characteristics and more flexibility in optical fiber arrangement would be desirable.
The ability to detect or monitor trace amounts of chemical species in situ is of great importance and generally difficult to do. Particular applications include environmental monitoring, pollution control, process control, public health and safety, clinical medicine and industrial monitoring. Groundwater, seawater and atmospheric (air) monitoring are very important. The improved reservoir FOCS would be particularly desireable for many of these applications.
The safety of drinking water and the ability of both sweet and sea water to support plant and animal life has become one of the world's key concerns. This, added to the already recognized air pollution problem further complicates the need for monitoring and corrective action. The list of substances that are to be monitored continually increases while, as clinical evaluations continue, the MAC (maximum acceptable concentration) is constantly decreasing. This means that the burden placed on analytical methods is quickly exceeding available specificity and sensitivity. Furthermore, as the accountability for analytical data becomes more demanding, the complexity of the instrumentation and the time to perform an analysis escalates accordingly. When these requisites are considered in light of the fact that there are hundreds of inorganic, organic and biological compounds that are potentially dangerous, toxic or carcinogenic and that there are also supplementary requirements such as pH, turbidity and the amount of coliform bacteria, the need for a simple method of analyses which can be adapted to many of the targets of interest becomes obvious.
The importance of safe and plentiful ground water supplies cannot be overstated. Domestic water quality is being threatened in many areas by the intrusion of toxic contaminants into the soil and the ground water from agricultural runoff of pesticides and herbicides; industrial discharge into lakes and rivers; and seepage from solid waste sites (landfills, storage lagoons, and waste piles). Unlike surface contaminants, which are quickly diluted, chemicals in the soil and ground water often remain highly concentrated both underground and in the water which flows from the faucet. The potential magnitude of the problem is enormous. It is, therefore, essential that an economical, practical water monitoring system be in place as soon as possible.
In order to provide adequate protection of water sources, methods of detecting low concentration of toxic contaminants are urgently needed. This includes measuring inorganic, organic and biological species. The public health, as well as the public's confidence in domestic water supplies, requires an early warning system so that prompt action may be taken to track down the sources of the contamination and to take appropriate steps to protect the public. In order to assure soil and water quality, the contaminants must first be identified. Presently, sophisticated state-of-the-art equipment and methodologies have been used for diagnostic investigation. Wells sometimes must be drilled for proper access to the vadose zone and ground water. Typically, gas chromatography and mass and atomic (absorption and emission) spectroscopy have been used in conjunction with special pumps and samplers to collect the soil and water to be analyzed. Unfortunately, present technologies are not suitable for continuous and widespread monitoring of groundwater contamination. Problems include the contamination of samples by well construction materials, degradation of sample integrity by most sampling techniques which could result in questionable data and make enforcement difficult, the high capital investment in complex equipment, and the need for highly skilled technicians. An improved reservoir FOCS which is accurate and uniform over a large number of sensors should be the basis of an acceptable, practical monitoring system.
The growing concern over depleted levels of oxygen and increasing levels of carbon dioxide and other trace gases on a global scale has warranted the need for improved methods for oceanic and atmospheric gas analysis and monitoring. The ocean flux process permits carbon dioxide to escape from the ocean and depleted oxygen to be replenished. According to recent predictions, "greenhouse" gases in the atmosphere are on a continuous rise and will result in a global warming of as much as 2.degree. to 3.degree. C. in the next 50 years.
The need for in situ monitoring of key chemical components in seawater is becoming increasingly more apparent. To develop a better understanding of the parameters which affect ocean flux requires measuring key chemical species as they simultaneously exist in order to obtain time series data. These components are currently measured by collecting water at the site using specially designed water samplers operated from a manned submersible or remotely operated vehicle. These samplers are made of titanium, are expensive, and time consuming to clean and maintain. The water samples are then brought to the surface and analyzed by conventional laboratory techniques to determine the results. FOCS have the potential to satisfy this monitoring need, which is unattainable with existing instrumentation.
There are presently a very limited number of FOCS systems available for commercial use in aqueous systems and virtually none available for use in seawater. Present day methods for the direct measurement of specific chemical parameters in seawater include deployed seawater probes for dissolved oxygen and pH that are based on electrochemistry. These Clark-type probes work either on the galvanic or polarographic principle and have a number of problems including slow response time, reproducibility, chemical and biological fouling of protective membranes, and other effects of high salinity on sensor performance and sensor lifetime. Fiber optic chemical sensors can overcome these problems and will offer many advantages, including batch fabrication at reasonable cost, expendability, small size, light weight and freedom from electromagnetic interference. In FOCS, signals are transmitted optically rather than electrically, which becomes advantageous when handling and deploying long lengths of sea cable in electrically noisy shipboard environments. A reservoir FOCS adapted to these types of measurements will make a groundwater or seawater monitoring system feasible.
Acidity (pH) of surface and ground water is caused by humic acid extracted from swamps or peat beds and by industrial pollution. Excessive acidity causes corrosion which results in many undesirable species entering into the water such as iron, lead and zinc and can be detrimental to fish life. A pH of 6.0-8.5 is both permissible and desireable in sweet water and 7.0 to 9.0 in sea water.
Arsenic in water is of great concern. Severe toxicity can exist after the ingestion of less than 100 mg of arsenic and chronic toxicity develops with even lower intakes. Arsenic enters the water system either from geological sources as arsenate and arsenite but it is industrial discharge that increases the arsenic content above safe levels. Although 0.1 ppm is the MAC (maximum allowable concentration) for arsenic, "virtually absent" is the desired level.
Bacteria count is used to determine the sanitary quality of water. Coliform bacteria are normally present in feces, soil and vegetation. It is important to distinguish between fecal and non-fecal coliform since it is the fecal species that is disease producing. The presence of more than 10,000 coliforms per 100 milliliters of water indicates a recent, and possibly dangerous, pollution event.
Benzene is one of the most dangerous toxicants. Not only is it a known carcinogen, but it also can cause irritation of the mucous membranes, convulsions and mood changes. Benzene sources include cigarettes, automobile fuels, and industrial processing. The result can be death from cancer or respiratory failure or blood disease. The MAC of 0.005 ppm is probably too high and complete absence is desirable.
Carbon dioxide has to be measured if there is any hope of understanding the ocean flux process. An in-situ measurement capability for dissolved carbon dioxide in the ocean is an important first step toward understanding the ocean flux process. This capability is needed by researchers engaged in ocean and global climate studies.
Chromium in drinking water is of concern because it is a suspected carcinogen. Chromium exists in two valence states, 3+ and 6+. Of these chromium (6+) is by far the most dangerous. It enters the drinking water from cooling towers, waste water plants, plating operations and the tanning industry. Although the MAC for chromium (6+) is 0.05 ppm, its presence in drinking water above 0.003 ppm indicates the presence of industrial pollution. The complete absence of chromium is the desired level.
Copper content in water should be "virtually absent ", although some small amount, 0.1 ppm, is needed for plant growth and body metabolism. If copper is totally absent from drinking water, nutritional anemia may appear in children. Concentrations greater than 0.1 ppm, on the other hand, diminish algae and plankton growth, but at the same time are toxic to several species of fish.
Cyanide is toxic to aquatic organisms at even the lowest levels. The toxicity is due to the liberation of hydrocyanic acid which inhibits oxygen metabolism. Natural waters do not contain cyanide. It comes, primarily, from industrial applications such as metal cleaning and electroplating baths, gas scrubbers and chemical synthesis. The MAC is 0.01 ppm but the desired level is absent.
Hydrazine is a violent poison having a strong caustic effect on the skin and mucous membranes. It can lower the metabolism by upsetting certain enzyme systems. The MAC for hydrazine is 5 ppm with a desireable limit of &lt;0.5 ppm.
Iron is not a potential health hazard but it has the ability to cause pipe encrustation and it causes aesthetic problems, i.e., rust spots on clothing and rust stains in sinks. It also makes the taste of drinking water objectionable. Iron enters the water system by leaching from natural iron deposits or from iron-containing industrial wastes. The soluble form is iron (2+) and the insoluble one iron (3+). The allowable limit is 0.1 ppm iron (2+) and 0.2 ppm iron (3+). The desired level is "virtually absent".
Lead in drinking water arises from many sources such as lead pipes and plastic pipes stabilized by lead. It is toxic to aquatic organisms and accumulates in the human bone structure when more than 300 micrograms per day are ingested. In many situations the actual concentration of lead present is masked by the precipitation of lead chloride and lead carbonate. The allowable lead concentration is 0.05 ppm with absent being the desired level.
Mercury may cause nausea, abdominal pain, vomiting, diarrhea and headache. Continuous exposure to mercury could result in inflammation of the mouth and gums, kidney damage, spasms and a change of personality. The MAC is 0.1 ppm with "virtually absent" being the desired level.
Nitrate in water indicates the final stage of water decomposition. High levels of nitrates may indicate biological waste material in the final stages of stabilization or contamination from the run off of agricultural fertilizers. Nitrate also enhances the excessive growth of algae. Since nitrite is changed to nitrates by certain bacteria, the MAC of 10 ppm is for the total nitrate/nitrite concentration. Amounts greater than this can cause moglobemia in infants ("blue babies"). The desired level is "virtually absent".
Oxygen is one of the most important measurements in both sweet and sea water because dissolved oxygen is the best water quality indicator. Dissolved oxygen (DO) is necessary to support all life in the marine environment and is, therefore, the most important water quality parameter. DO concentration is controlled by a process known as oxygen demand. Oxygen demand materials require oxygen for degradation which results in a depletion of ambient DO levels, thereby depriving marine organisms. Man made wastes, such as sewage sludge and other forms of organic wastes, are examples of oxygen demand materials. These wastes are also known to cause increased nutrient loading resulting in excessive marine plant growth. It is therefore essential to control the amount and type of waste materials dumped into marine waters and to routinely measure and monitor dissolved oxygen to ensure that adequate levels are available to support marine life. Conditions of low dissolved oxygen often occur in highly populated estuarine and coastal areas. Drinking and industrial water should contain at least 4 ppm oxygen but, in general this must be higher to sustain aquatic life. 8-15 ppm is adequate in both sweet and sea water while air - saturated oxygen is the desired level.
Phosphate is important because in excess it leads to atrophy in lakes. It is also necessary to monitor for phosphate in boiler and cooling towers because it encrusts on the walls. Phosphate gets into the water system from agricultural run off, biological waste, corrosion control materials, detergents and surfactants. The MAC is 50 ppm. Inorganic phosphate must be distinguished from the very toxic organic phosphates which are present in pesticides and chemical agents. These are enzyme inhibitors and can cause death. The MAC for organic phosphates is 0.1 ppm with completely absent being the desired level.
Selenium is extremely toxic to humans and animals. It causes inflammation of the lungs and disturbs the digestive and nervous systems. Selenium is a known carcinogen. Its main sources in water are industrial waste and the dissolution of selenium-containing soils. The MAC is 0.01 ppm with complete absence being desireable. Sulfates themselves are not toxic. They do, however, increase the solubility of other very toxic compounds such as lead. They may also lead to diarrhea by forming the laxatives magnesium and calcium sulfates. The MAC for sulfate is 250 ppm with &lt;50 ppm being desireable.
Sulfides are extremely toxic, especially hydrogen sulfide gas. Collapse, coma, and death from respiratory failure can occur within seconds of its intake. It is as toxic as hydrogen cyanide. Fortunately, its obnoxious odor is detectable long before toxic levels are reached. From a pollution stand point hydrogen sulfide is the by-product of the anaerobic decomposition of organic matter and indicates serious water contamination. The MAC is 0.01 ppm with complete absence being the desired amount.
Sulfur dioxide irritates the respiratory system and could cause bronchitis and asphyxia. It may also be an eye irritant causing conjunctivitis. The MAC is 10 ppm with &lt;0.1 ppm being desireable.
Trichloroethylene (TCE) heads the U.S. Environmental Protection Agency (EPA) list of hazardous (toxic, carcinogenic, etc.) compounds and the organic chlorides, as a group dominate the top ten (10) most frequently found dangerous compounds. TCE is of particular concern because it forms the carcinogen vinyl chloride in water. Moreover, it is estimated that about 23 million people in the United States are exposed each year to TCE levels ranging from 0.5 to 5 ppm even though 0.005 ppm is the MAC and absent is the desired concentration.
The EPA has requirements or is formulating regulations that will require that the above species, as well as others not listed, be continuously monitored. To accomplish this task requires that a device be developed that is both inexpensive to purchase and operate and that can give reliable results in the hands of a moderately trained field technician. Thus the ability to perform the monitoring task (qualitatively and quantitatively) is significant.